The present invention pertains generally to integrating radiation detectors and more particularly to high resolution imaging detectors.
Generally, two types of imaging radiation detectors have been used in the past. The first of these, the gridded ionization chamber, is capable of generating two-dimensional information but is incapable of providing gain. In essence, the gridded ionization chamber operates by providing a cathode plane and a two-dimensional anode array. Normally, the anode array is held at near ground potential to increase signal to noise ratio. Since the cathode is held at some negative dc potential, substantially straight field lines are produced between the cathode plane and anode array. When radiation impinges upon a detector gas disposed between the cathode and anode, charge carriers are generated which drift along the substantially straight lines. Negative charge from electrons are accumulated on the anode array to provide an intensity image of the radiation. Since electric charge carriers are only generated by the interaction of the radiation with the detector gas, gridded ionization chambers are normally only useful with high intensity radiation which is capable of producing a large number of charge carriers.
To overcome these problems the integrating multiwire proportional chamber utilizes a cathode plane which is held at a predetermined negative potential. The anode plane constitutes a series of parallel wires which are held at a predetermined positive potential. By increasing the potential drop between the cathode and anode, charge carriers which are produced by the interaction of the radiation with the detector gas, are accelerated by the electric field potential between the cathode and anode. The acceleration of electrons towards the anode wire causes these electrons to collide with, and ionize, detector gas molecules, thereby releasing new electrons, which, in turn have more collisions, so as to produce cumulative ionization and thereby increase the number of charged particles, i.e., electrons collected on the anode wire plane. This gain in detector response can increase the number of charged particles collected by a factor of 10.sup.5. Consequently, much better signal to noise ratios can be achieved with multiwire proportional chambers and much lower intensity radiation can be imaged.
However, several problems exist with the integrating multiwire proportional chamber. It has been found that the minimum practical anode wire separation is approximately 2 mm which consequently limits the position resolution of the multiwire proportional chamber. Secondly, multiwire proportional chambers are limited to one-dimensional position imaging since the current produced by the collection of electrons on the anode multiwire plane is integrated and amplified separately for each anode wire. Although orthogonal anode wire planes can be disposed on two sides of the cathode plane, only one-dimensional information in two directions can be achieved in the multiwire proportional chamber. Moreover, use of a two-dimensional pattern on a single plane, such as a checkerboard pattern, will not cause avalanching of charged carriers since only wire planes using wires of a specified diameter are capable of producing the avalanche effect, i.e., the cumulative multiplication of charged carriers.
Consequently, it is desirable to produce an imaging radiation detector which provides true two-dimensional information and utilizes the avalanching effect to provide gain in the system to allow the system to image low intensity radiation with increased signal to noise ratio.